New transmission schemes for extending a transmission rate are required according to an increase in an amount of communication due to the widespread use of mobile communication terminals such as smart phones and tablets. As one of such schemes, a Long Term Evolution (LTE) time division duplex (TDD) scheme is adopted in addition to a conventional LTE frequency division duplex (FDD) scheme. The FDD scheme is a duplex scheme in which frequency bands for use in an uplink (hereinafter referred to as “UL”) and a downlink (hereinafter referred to as “DL”) are provided separately. The FDD scheme is a scheme in which a transmission rate is increased by independently performing UL and DL transmissions. However, because frequency resources are finite, it is difficult to newly allocate a broadband for the FDD scheme. Thus, an LTE TDD scheme of dividing UL and DL with respect to time by using the same frequency band is beginning to be adopted. Because the TDD scheme requires half a band as compared with the FDD scheme, it is possible to effectively utilize frequency resources therewith.
On the other hand, as one method of transmitting radio waves transmitted from a wireless device such as a mobile phone base station to a plurality of dead zones which radio waves do not reach, a distributed antenna system (DAS) for drawing a slave station for a base station in each dead zone in a wired manner is provided. The DAS can not only eliminate the dead zone but also can reduce an installation space for a base station. Thus, DASs are being actively used in zones which radio waves do not reach such as the inside of buildings, underground malls, and tunnels.
Here, if the LTE TDD scheme is applied to the DAS, it is necessary to perform switching of transmission/reception using UL and DL. If there is a deviation between a switching timing of the transmission/reception of the DAS and a switching timing of the LTE TDD scheme, UL and DL interfere with each other and communication quality deteriorates. Due to this mutual interference, the transmission/reception of a control signal between a base station and a user terminal is obstructed, so that communication of the user terminal is disabled. Thus, in the LTE TDD scheme, a guard time is provided between UL and DL in order to prevent mutual interference between UL and DL. Mutual interference between UL and DL can be prevented by performing switching of the transmission/reception of the DAS during this guard time.
In order to perform switching of the transmission/reception of a DAS during the guard time, it is necessary to detect the beginning of an UL signal or a DL signal. If the beginning of a signal can be accurately detected, it is possible to perform transmission without affecting the communication quality even if the signal has a short guard time. Conventionally, the beginning of the UL signal or the DL signal is detected based on a switching timing between UL and DL estimated on the basis of configuration information in the LTE TDD scheme. However because it is necessary to decode a received radio signal to acquire the configuration information in the LTE TDD scheme, an apparatus constituting the DAS becomes complicated.
Also, as another detection method, the beginning of the UL signal is detected by estimating a radio signal of a user terminal connected to a macro base station that can be an interference source on the basis of an output of a detector. However, because the output of the detector generally has a variation of about several microseconds, it is difficult to detect the beginning of the signal with high accuracy. As described above, it has conventionally been difficult to detect the beginning of the UL signal or the DL signal with a simple configuration and high accuracy.